Lightweight composite materials are being used more extensively in the aerospace industry for both commercial and military aircraft and other aerospace vehicles, as well as in other industries. The structures using these composite materials may be formed using multiple plies or layers of material that may be laminated together to form a high strength structure. These structures may undergo further machining processes during manufacturing and assembly of vehicles (drilling, cutting, countersinking, shimming, fastener removal, etc.); as well as further processes related to flight and ground operations (maintenance, repair, retrofit, or overhaul); and damages from an impact or other causes. The ability to characterize the condition of a laminated structure prior to delivery, repair, refurbishment, return to service, salvage, etc. may be required.
It would be advantageous to have a tool that can scan inside holes or cut outs in the structure to assess the condition of the composite in the vicinity of the drilled hole or machined edge, especially if a first delamination was identified with conventional longitudinal (L-wave) pulse echo ultrasound. Conventional pulse echo ultrasound is often reflected by the first delamination, and as a result it limits the operator from acquiring data on the delamination lying beneath it in a stratum of plies. Another difficulty is that conventional transducers with sufficient energy and appropriately narrow beam spread, are physically too large to be incorporated in a transducer wedge that can be inserted in a small hole for inspection of the structure open to the hole.
A further disadvantage of existing solutions is that the maximum depth of the damage is unknown without repeated cycles of inspecting and sanding which results in removal of overlying material. Since repairs often require a 30:1 taper on the sanded-out crater or “scarf”; small damage that is only a few plies deep can result in a large repair surface area that intrudes on other structures, and requires major disassembly. Large repairs also may result in a large area exposed to heat when the repair plies are cured. Further, the time spent in repeated cycles of inspection and sanding to establish the eventual repair size can be considerable. Often, intermediate inspections reveal damage that was not discernible prior to the start of damage removal operations. As a result, the repair scope changes mid-way through the process and engineering must reconsider the overall size of the repair and its impact on surrounding structure.
In addition, there is a need for “one shot” inspection methods to assess the condition of a structure. For example, having a tool to locate and map damage or delamination in plate-like structures, down through multiple plies open to the drilled holes or machined cut-outs, would allow an operator to quickly inspect the structure around the drilled hole. Current methods do not allow inspectors to characterize the condition in “one shot process”, and thereby hampers early development of a plan that encompasses the magnitude of the subsequent repair.
Visual and remote visual inspection methods are also used to try to image delamination in damaged areas and drilled holes, but experience has shown that some delamination are missed by visual inspection, while being detected by ultrasonics. This may be due to tightness of the delamination, glare conditions in the hole, or drilling dust filling the delamination surface.
It would therefore be desirable to provide improved techniques for inspecting a structure. The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.